The present invention relates to analog signal processing systems, and particularly to systems for changing the frequency of analog signals in a real-time manner. For purposes of example, the invention is described below with respect to two types of systems, namely as an aid to hearing by a person having a limited-frequency audibility, and as a means for communicating audio signals in a manner non-intelligible except by a properly equipped receiver. It will be appreciated, however, that the invention could also be used in other applications, for example in the analysis of transient signals.
With respect to the first of the above-mentioned applications, it is to be noted that the upper limit of human audibility is usually about 15-20 kHz, but many persons have a much more limited range and are not able to hear tones within this frequency range. The invention, as to be described more fully below, is particularly useful in this type of application for the real-time reduction of the frequency of audio signals to a range which a person, having such a hearing deficiency, is capable of hearing.
With respect to the second of the above-mentioned applications of the invention, various secret communication systems have been proposed for scrambling information transmitted, e.g. by a radio transmitter, so as to render the information unintelligible except by a properly equipped receiver. Such scrambling systems are quite costly and therefore have limited application. The invention may be used for simplifying and reducing the cost of secret communication systems.